Computer disk drives store information on magnetic disks. Typically, the information is stored on each disk in concentric tracks that are divided into servo sectors and data sectors. Information is written to or read from a disk by a transducer head, mounted on an actuator arm, capable of moving the transducer head radially over the disk. Accordingly, the movement of the actuator arm allows the transducer head to access different tracks. A disk is rotated by a spindle motor at a high speed, allowing the transducer head to access different sectors within each track on the disk. The transducer head may include integrated read and write heads.
In a typical computer disk drive, error correction code (ECC) is commonly used to assist in the reliable retrieval of stored data. In general, error correction code allows data that may have otherwise been lost to be reconstructed. However, serious defects in the media used to store the data can overwhelm the ability of the error correction code to rebuild lost data upon retrieval. Furthermore, error correction code requires that data be stored on the disk that is in addition to the user data. Therefore, error correction code occupies storage space that could otherwise be used to store user data. Furthermore, as the ability of an error correction code scheme to retrieve lost data is increased, the storage space required to store the additional information required by the error correction code generally increases. For all these reasons, the ability of error correction code to rebuild lost data is typically limited.
In order to help prevent delivery of computer disk drives having defects in storage media to end users, disk drives are typically tested for such defects. According to a typical testing procedure, data is written to the storage media in a test pattern. The test pattern is then read from the storage media and the results of the read operation are compared to the expected results. For example, a signal produced in the channel of a hard disk drive as a result of reading the test pattern may be periodically sampled, and the amplitudes of the samples may be compared to the expected amplitudes. A signal indicating the detection of a defect may be generated if a sampled value is less than the corresponding expected value. For further information regarding a method and apparatus used to detect flaws in storage media, see U.S. patent application Ser. No. 09/848,089 filed May 2, 2001, entitled “METHOD AND APPARATUS FOR FLAW DETECTION IN SYNCHRONOUS SAMPLING (PRML) READ CHANNELS USING POST PROCESSED DIGITAL FILTERS” to Curtis Egan, and assigned to the assignee of the present invention, the entire disclosure of which is hereby incorporated by reference.
In response to receiving an indication that an area of the storage medium contains a defect, the controller of the hard disk drive may spare, or mark as unsuitable for storing data, the affected area. For example, the data sector containing the defect, or all of the data sectors in the area between the hard sectors in which the defect is located, may be spared. However, the sparing of portions of the storage media diminishes the storage capacity of the hard disk drive. Therefore, it is desirable to avoid unnecessarily sparing portions of the storage media. In addition, it often is unnecessary to spare a portion of the storage media in response to the detection of an isolated defect. This is because the error correction code is often sufficient to allow for the reliable storage and retrieval of data in areas of the storage media that contain relatively few defects. Therefore, such areas need not be spared in order to provide a disk drive capable of reliably storing and retrieving data.
In addition, it is desirable to locate defects on storage media within a reasonably small area of the media, so that the spared areas may be as small as possible. However, conventional systems that allow information regarding the location of defects to be stored have typically required that the specific address at which the defect was detected be stored, at least temporarily. Such methods therefore require that additional memory be provided that is capable of storing such information for later analysis. In addition, the analysis of defect location data to determine its location on the disk in proximity to other defects requires processing power and time, as well as implementing software or firmware, and cannot be performed in substantially real time.
For the above stated reasons, it would be desirable to provide an improved method and an apparatus for determining whether flaws detected in storage media require sparing. In addition, it would be advantageous to provide such a method and apparatus that was capable of determining the density of defects per unit area of the storage media, without requiring specific defect location information to be stored. Furthermore, it would be advantageous to provide a method and an apparatus for assessing whether an area of storage media should be spared that could be implemented as part of the firmware of a computer disk drive, or of software controlling the operation of a computer disk drive. Furthermore, it would be advantageous to provide such a method and apparatus that are reliable in operation and that are relatively inexpensive to implement.